1. Field of the Invention
The present invention is directed to the employment of an amorphous, magnetostrictive alloy in monitoring or identification systems for producing magneto-elastic tags that can be deactivated by removing a pre-magnetization field.
2. Description of the Prior Art
Magneto-elastically excitable tags usually contain monitoring strips that are composed of an amorphous alloy with high magnetostriction. PCT Application WO 90/03652 discloses the employment of amorphous alloys containing nickel in addition to iron for monitoring systems with mechanical resonance.
Alloys with magnetostrictive properties can be employed, for example, in identification systems for tags. The magnetostriction is exploited in order to place a strip of this alloy into oscillation by means of an alternating field acting on this strip. When the alternating field is deactivated, the strip, which continues to oscillate, generates magnetic field changes that a pick-up coil converts into induced voltage pulses. An evaluation of these voltage pulses reveals whether a strip of the oscillating material having a specific strip length is contained in the field.
An item can thus be identified by applying a strip of a particular length thereto, the length serving as an identifier, or an anti-theft security system can also be based on this principle, whereby only magnetized strips of a specific length are attached to the goods and the presence of such a strip is detected by the coil system for field excitation and for pick-up of the magnetic oscillations after every excitation time span.
When a strip of amorphous magnetostrictive material is exposed to a magnetic field, then the magnetostriction causes a change in the length of the strip. This dependency, however, is not linear but is dependent on the dimensions of the strip and on the size of the magnetic field. When the magnetic field is boosted in equal steps given a specific strip, then one finds that only small changes in length initially occur, then the changes in length become greater with increasing steps of the magnetization boost, and then no further change in length ensues upon the occurrence of saturation, despite a magnetic field that continues to be boosted in steps.
The effect of this property is that such a strip can be excited to mechanical oscillations when it is exposed to a pre-magnetization field whose size results in a great change in length given a uniform change of the magnetic field. A further effect of the change in length ensuing due to the magnetic field is that the length of the strip changes in this region without a tensile stress acting on the strip.
The modulus of elasticity of the material is the determining factor for the resonant frequency of the oscillation given mechanical oscillation of a strip. The force required for a specific change in length becomes greater and the resonant frequency of the oscillating strip becomes higher, as the modulus of elasticity increases. An additional change in length, however, ensues due to the influence of the magnetic field without a force being necessary. The material thus acts as though it had a lower mechanical modulus of elasticity than it really has.
The result is that the resonant frequency given excitation by an alternating magnetic field becomes lower with increasing pre-magnetization than it is without pre-magnetization. A strip that oscillates at a specific resonant frequency with high signal amplitude with a given pre-magnetization will oscillate substantially less given excitation with the same frequency when the pre-magnetization field is removed, because the resonant frequency is thereby boosted and the exciting frequency and the resonant frequency no longer coincide.
The removal of the pre-magnetization field also results in a change of the magnetic field now only results in a relatively slight change in the length of the strip, so that the signal height also significantly decreases without pre-magnetization field.
Together, the two factors cause a mechanical oscillation of the strip to be suppressed upon removal of the pre-magnetization field. It is thus possible to deactivate an anti-theft security strip composed of this material by removing the pre-magnetization field.
This is achieved in the case of anti-theft security strips by, for example, a demagnetizing magnet connected to the strip. In other systems wherein the pre-magnetization field is in part generated by a coil in the examination area, the oscillation can be suppressed by turning off this pre-magnetization field.
Despite these relationships, a reliable deactivation within the same time required for a reliable response cannot always be achieved with known pre-magnetization techniques. The reason for this is that the earth""s magnetic field also acts on the monitoring strip in addition to the pre-magnetization field, and build-up of tolerances that influence the resonant frequency must be taken into account given mass production of monitoring strips.
When the monitoring strip is rotated in the earth""s magnetic field, this will increase the pre-magnetization at one end region or strip-half and decrease it in the other. This results in a natural fluctuation of the resonant frequency. The monitoring apparatus, however, must then be set such that these fluctuations of the resonant frequency do not lead to the failure to generate an alarm under proper circumstances, while still insuring that the resonant frequency changes to such an extent upon removal of the pre-magnetization field is certain that an alarm can no longer be triggered.
It is an object of the present invention to specify an alloy for employment as monitoring strip in monitoring or identification systems that responds reliably under true alarm-causing conditions but which can be reliably deactivated. This requires an alloy which exhibits only a relatively slight change in its resonant frequency caused by superimposing the earth""s magnetic field on the pre-magnetization field, but exhibits a considerable change in its resonant frequency when the pre-magnetization field is removed.
The above object is achieved in accordance with the principles of the present invention in an amorphous alloy having the formula Fea Cob Nic Six By Mz employed as monitoring strips for mechanically oscillating tags, for example for anti-theft protection, together with a source of a pre-magnetization field in which the strip is disposed so as to place the strip in an activated state. In the formula, M denotes one or more elements of groups IV through VII of the periodic table, including C, Ge and P, and the constituents in at % meet the following conditions: a lies between 20 and 74, b lies between 4 and 23, c lies between 5 and 50, with the criterion that b+c greater than 14, x lies between 0 and 10, y lies between 10 and 20, and z less than 5 with the sum x+y+z being between 12 and 21. These alloys have a resonant frequency associated therewith and when passed through an alternating field whose alternation frequency coincides with the resonant frequency, a pulse having a signal amplitude is produced. These alloys can be deactivated by removing the pre-magnetization field, which causes a change in the resonant frequency and the resulting signal amplitude. These alloys exhibit a change in resonant frequency and signal amplitude due to changes in the orientation of the tag in the earth""s magnetic field which is smaller than the change occurring upon removal of the pre-magnetization field, so that the tag can be reliably deactivated.
In addition, the invention achieves other objects that are important, in particular, for monitoring systems for anti-theft protection. In an anti-theft monitoring system that is based on mechanical resonance, the monitoring strip is deactivated by demagnetizing a magnet attached to the strip. Monitoring strips which are thus connected to the goods, however, could undesirably trigger a false alarm in a monitoring zone operating according to the harmonics method. In such monitoring systems, monitoring strips are discovered in an examination field by evaluating the harmonics of the exciting alternating field that they generate.
It is important that goods with magneto-elastically excitable tags that have already been deactivated do not trigger an alarm. This is achieved with the inventive tags by setting a flat magnetization loop without remanence discontinuities by a thermal treatment in a transverse magnetic field,